The invention relates to the field of fish elevators which use the principle of the Archimedes' screw to raise fish from a body of water to a storage area.
The Archimedes' screw has been applied to the raising of fish from a body of water for the purpose of transportation, sorting and the like. For efficient operation of such a device it is important that sufficient dewatering of the lifting screw be provided to avoid carrying too much water up with the fish, and also that the structure of the entry portion be such that fish are not repelled. For example, Canadian patent no. 1,093,400 issued January 13, 1981 to Faivre discloses an Archimedes' screw adapted for lifting fish. This device has two screw sections rotating on a common shaft - a lifting screw which is a solid helix, and a catching screw which is a lattice-work helix. The fish are transferred from the catching screw to the lifting screw by means of a deflector formed in the cylindrical drum around the lifting screw. The lattice-work screw portion serves to dewater the transported fish. Typically the latticework must be metal and so the sheath around the entry portion is also of metal, adding to the cost of construction.
A problem with existing designs results from the fact that fish prefer to swim into a current and are frightened by an area of low water pressure. Further, the lattice-work helix design is expensive to mass-produce. Also, since it draws no water at its point of entry, a longer auger is needed for a particular length of lift.